fix: remove model field from lettabot config, add lettabot model command

The model field in lettabot.yaml was redundant and misleading -- the model
is configured on the Letta agent server-side, and lettabot shouldn't
override it. Users were confused seeing a model in their startup log that
didn't match the actual model being used.

Changes:
- Remove `model` from `LettaBotConfig.agent` (made optional for backward compat)
- Remove `model` from `BotConfig` interface and `bot.ts` createAgent() calls
- Remove `model` from `main.ts` config construction and startup log
- Stop saving `model` to lettabot.yaml during onboarding
- Stop mapping `agent.model` to `MODEL` env var in config/io.ts
- Add `getAgentModel()` and `updateAgentModel()` to letta-api.ts
- Add new `src/commands/model.ts` with three subcommands:
  - `lettabot model` - interactive model selector
  - `lettabot model show` - show current agent model
  - `lettabot model set <handle>` - set model directly
- Wire up model command in cli.ts with help text
- Update docs/configuration.md, lettabot.example.yaml, SKILL.md

Model selection during `lettabot onboard` is preserved for new agent
creation -- the selected model is passed to createAgent() but is NOT
saved to the config file.

Fixes #169

Co-authored-by: Cameron <cpfiffer@users.noreply.github.com>

lettabot.example.yaml

@@ -15,10 +15,8 @@ server:
 
 agent:
   name: LettaBot
-  # Model to use:
-  # - Free plan: zai/glm-4.7, minimax/MiniMax-M1-80k
-  # - BYOK: lc-anthropic/claude-sonnet-4-5-20250929, lc-openai/gpt-5.2
-  model: zai/glm-4.7
+  # Note: model is configured on the Letta agent server-side.
+  # Select a model during `lettabot onboard` or change it with `lettabot model set <handle>`.
 
 # BYOK Providers (optional, cloud mode only)
 # These will be synced to Letta Cloud on startup